The science of analytical chemistry and particularly simple to use solid state test devices using analytical chemistry principles has made dramatic progress over the past decade or so. At one time such devices simply gave an indication of the presence of a substance or a gross condition of the fluid being analyzed, such as, for example, the use of litmus paper to determine if the fluid was acidic or basic. Now such devices can give answers which are as precise, specific and sensitive as those obtained using laboratory procedures and conditions. Moreover, such devices can quite often be used without accompanying instrumentation which permits their use in the field or "on-site" to give instant answers. This obviously eliminates the need for preserving sample integrity, simplifies record keeping and allows the user to take rapid corrective measures.
Present simple to use solid state testing devices usually take the form of either (1) a paper matrix pad impregnated with a reagent which develops a color when the entire pad is immersed in or contacted with an analyte in solution or (2) a reagent impregnated bibulous or porous matrix which is enclosed in a fluid impervious sheath or covering which restricts the flow of fluid being tested to a defined opening, usually an end portion of the sheath. In use, this latter type device is contacted with the fluid being tested such that the opening is exposed to the fluid which wicks up or into the bibulous matrix by capillary action (or is pulled or pushed into and through the porous matrix using applied force such as pressure or vacuum), wherein an analyte or a conversion product thereof in the fluid reacts with the reagent in the matrix to form a localized reaction product giving a visual response as the fluid moves through the matrix.
The pad type reagent strip device is usually made quantitative or semi-quantitative by using a chromogen in the reacting mixture which responds proportionally to the amount of analyte in the fluid being tested. This response can either be read visually by comparison to a developed color chart or by inserting the pad in a reflectance photometer which electronically "reads" the amount of color formed and interprets this as a quantitative value. The photometer obviously gives the analysis a higher degree of precision and sensitivity.
The sheath enclosed reagent incorporated matrix (SERIM) type device is interpreted by measuring the reaction of the analyte with the reagent as the fluid moves through the matrix. This is accomplished by effecting a localized visual change in the matrix by such reaction until the analyte is exhausted from the moving front of the wicking fluid. The sheath enclosing the matrix usually contains a means such as spaced marking lines and a numerical scale for measuring the extent of the chemical reaction. The number opposite the visual change is then compared to a calibration chart to give a quantitative result; however, if the device can be fabricated consistently, the numbers on the device can be the actual values for the concentration of analyte in the fluid being tested.
It has long been known that silver dichromate reagent systems incorporated in a porous matrix are excellent devices to test for the presence of halides such as chloride ion in a wide variety of Health related and commercially important substances such as body fluids, foods and beverages, industrial processing fluids and building supplies. Basically the devices are contacted with the fluid being tested and any color change in the reddish brown color of the test reagent matrix is used to judge the presence and amount of halide present in the fluid. It has, however, recently been found that complex alkaline fluids such as wet concrete or mortar contain interfering substances which react with the reagent system and cause a darkening effect in the matrix, thus preventing the facile reading of the color change. It has been particularly determined that alkaline fluids containing relatively high concentrations of hydroxyl ions suffer from the disadvantage that such hydroxyl ions react with the silver dichromate to form browninsh-black silver hydroxides which ultimately form black oxides and consequently prevent or obscure the detection and measurement of any white silver halide which causes the change in color in the silver dichromate reagent system.